Shallow drilling hazards in carbonate formations are well known to present potential problems in exploration and developmental drilling and can represent a significant risk to the exploitation of hydrocarbons. Shallow formation carbonates are subject to the presence of groundwater and dissolution, creating void air spaces (caves) of varying and highly irregular dimensions. Some of these voids collapse totally or partially, while others remain intact. If a drill bit and drill string encounter such a karst feature, there is an immediate loss of circulating fluid, and there also can be a bit drop through the void space of the karst. This can result in the total loss of the well at great expense.
Production three-dimensional seismic data is normally gathered and employed for the imaging of seismic reflection data for targeted and prospective reservoirs. This data is then analyzed by seismic interpreters, sometimes using three-dimensional visualization techniques, to interpret and map these reservoirs for the purpose of locating areas of trapped hydrocarbons for subsequent exploitation by drilling. For example, the seismic reflection data can be displayed in a three-dimensional cube or a portion of a cube on a screen in a map view of the data in the prior art as shown in FIG. 1. However, such displayed reflection data can show little continuity and so cannot aid in detecting shallow drilling hazards.
The oil industry has for some years recognized the desirability, if not the necessity of locating and avoiding shallow drilling hazards. These hazards to drilling are very time-consuming to traverse with the drill bit and therefore expensive, and represent a potential danger to drilling crews. Most industry efforts to solve the problem that have been published and, in some cases patented, are associated with exploration in marine offshore environments. Shallow subsurface voids and the potential for mudslides can endanger the drilling operation. Further problems can be caused by shipwrecks and other man-made obstructions. It is also possible for localized zones of natural gas under pressure to exist in very shallow rock strata that would pose both a drilling risk of blowout, as well as a structural risk to the platform.
In marine exploration and development programs today, it is common for both corporations and governments to require the acquisition of a seismic hazard survey that is usually two dimensional for a planned drilling location. This requirement is particularly appropriate where large and expensive drilling platforms must be built and positioned over an area to be drilled. The sea floor must be able to sustain the forces of drilling equipment and operations. Should failure occur, it would result in the potential loss of the platform and associated equipment, risk the lives of operating personnel and the loss of millions of dollars in capital investment. The environmental risks are obvious and significant. These marine hazards can be detected by the utilization of streamer seismic data and by the careful processing of the signal to preserve the phase and relative amplitudes of the reflected arrivals in the shallow section below the water bottom. Drilling hazards can often be detected using a method such as that described in U.S. Pat. No. 5,555,531, that employs three-dimensional seismic data in a marine environment.
To date, all known efforts to locate these karsted features in seismic land data in carbonate environments have relied on the use of seismic reflection data. The results have been limited or poor. For example, FIG. 2 illustrates a computer screen displaying a two-dimensional visualization of seismic reflection data in accordance with the prior art at a point of lost drilling fluid circulation, with the three vertical lines in the data representing well bores. As shown in FIG. 2, the use of conventional seismic reflection data makes it quite impossible to accurately detect any lost circulation, as the noise of the data overwhelms the few traces occupying each bin of data. While this reflection approach will generally work in a marine environment, it will not work in a high-noise land setting.
Previous attempts to detect shallow hazards include the so-called seismic-while-drilling (SWD) method. The goal was to gain the ability to look ahead of the bit while drilling is underway using the descending drill bit as an acoustic source, and in conjunction with surface-located receivers. For example, in U.S. Pat. No. 6,480,118 a seismic-while-drilling method is described that generates seismic data useful for looking ahead of the drill bit which is employed as the acoustic source. The processed look ahead data is used to maximize the drilling penetration rate based on the selection of more effective drill bits. The method purports to be useful in minimizing the risks of encountering unanticipated drilling hazards. The SWD method suffers the drawback that the well is already positioned and drilling is underway, so that the well might be placed in a disadvantageous location, without any practical alternative but to keep drilling.
A second approach has been to employ reflection seismic data in an effort to map these karsted features. To date, this method has not been entirely successful. The reason for the lack of success is the relatively poor sampling of reflections in the very shallow portion of the seismic prestack record. Absent a very high-resolution survey, which for large drilling programs would be prohibitive in terms of time and cost, there is no apparent method using reflection data that can be improved sufficiently to reliably identify the shallow hazards.
Other proposals and efforts to employ different types of data, such as ground-penetrating radar (GPR) have not proved practical, since penetration into the karsted subsurface is inadequate.
For example, the method disclosed in U.S. Pat. No. 4,924,449 employs reflected energy from a highly specific location using a positional sub-surface transducer array. While useful in marine environments, it is not useful in a land setting.
A survey of the patent literature has not revealed a satisfactory solution to the problem.
U.S. Pat. No. 6,593,746 describes a method for radio-imaging underground structures for coal beds, with subsequent analysis performed using Full-Wave Inversion Code (FWIC). It can be used in mining operations where transmitters and receivers are placed in the passageways of mines, conditions which are not present in oil and gas exploration operation.
U.S. Pat. No. 6,501,703 describes a method utilizing first arrivals of seismic waves that are used to calculate and correct for time statics.
U.S. Pat. No. 5,757,723 describes a method for seismic multiple suppression using an inverse-scattering method for reflection and transmission data only.
U.S. Pat. No. 6,473,696 describes a method for obtaining and using seismic velocity information for the determination of fluid pressures for use in the analysis of fluid flow in reservoirs, basin modeling and fault analysis.
U.S. Pat. No. 5,671,136 describes a process that removes the refraction information present in the data, and then uses the seismic reflection data to define hydrocarbon-bearing strata, aquifers and potential drilling and mining hazards, utilizing visualization.
A method specifically directed toward the detection of drilling hazards in marine environments using high-resolution three-dimensional seismic data based on reflection data that has been processed to retain broad bandwidth is disclosed in U.S. Pat. No. 5,555,531. It employs reflection seismic data analysis identifying mud slides, shipwrecks, salt structures, mud flows and fluid expulsion features in deeper water environments, i.e., water depths of 800 feet or greater.
Seismic data is produced when a seismic compressional acoustic waveform is produced at the surface by a source such as dynamite or a mechanical source, e.g., a device such as that sold under the trademark VibroSeis™. The waveform spreads as a spherical wave propagation into the earth where it is both reflected and transmitted through rock strata in the subsurface. The reflected energy returns to the earth's surface as reflected waves, where it is recorded by receivers, such as geophones, that have been positioned on the surface at predetermined points displaced from the source.
When a source generates a waveform, it spreads in depth (Z direction) and laterally (X and Y directions). When a waveform spreads at a certain angle (the critical angle), it bends or refracts, and travels along a rock interface rather then through it. This portion of the wave energy is returned to the receivers as a refracted wave.
As noted above, in relatively shallow rock strata, karsts can exist. Geologically, they are produced by the dissolution of rock, i.e., the chemical reaction between carbonates and water. These subterranean caves or voids can be highly irregular in shape and size. In the case of larger karsts or as a result of increases in overburden forces, these voids cannot support the weight of the rock strata above and they collapse on themselves. These collapses can be unconsolidated, that is, there remain a series of much smaller karsts; or they can be consolidated, for example, as a result of further collapses.
When a refracted wave travels along a relatively homogeneous rock interface, the waveform will do so at a specific velocity and travel back to the receivers on the surface where they are recorded at a certain time, frequency and amplitude over a predetermined sampling interval. However, when the refracted waveform encounters a void or a heterogeneity in its path, the waveform is disturbed and the resultant amplitude and/or frequency of the wave returned to the receivers is abnormal.
The situation is very different on land, however, although the risks and dangers of near-surface hazards are similar. These include, but are not limited to, the loss of the borehole, damage to well structures and equipment, blow-outs, environmental damage and lost drilling fluid circulation. The adverse effects of an unexpected encounter with shallow drilling hazards can be elucidated as follows:
1. Lost Circulation of Drilling Fluids                A. Any sudden loss of the circulating drilling fluid incurs both a monetary loss and an increase in mechanical risk to the equipment.        B. If hazards could be identified prior to drilling, the drilling engineers could plan the mud injection program accordingly, which at present they are unable to do. This would result in improved use of material and monetary savings during drilling.        
2. Unexpected Drill Bit Drops                A. A drop through a void or karst can result in mechanical damage to the drill string and bit.        B. The drill string can become stuck in the hole, resulting in the loss of the borehole, in which case the entire well must be redrilled at enormous costs in time and money.        
3. Personnel Safety Issues                A. If shallow karsted zones are unknown to drilling personnel, a bit drop can be hazardous to workers on the drilling platform floor.        B. Under some circumstances, the rig itself can be damaged if the drill string drops through the drilling floor.        
The problem with a land environment, particularly one characterized by shallow carbonates and anhydrites, is that using reflection data will not work as it does in the marine environment. The reasons for this include:
1. In normally-acquired seismic data, the survey and dimensions are designed for deeper targets which possess commercial potential for hydrocarbon accumulation. These surveys are therefore not sampled adequately in the spatial domain closest to the earth's surface.
2. Reflection land seismic information in the shallow subsurface (above about 1,000 feet) will be muted in the processing of the data. Later, the data recorded at each time sample will be corrected for normal moveout and stacked to suppress random noise. The problem is that, in these shallow zones, there is usually inadequate sampling in offset to statistically cancel out the noise.
As used herein, the terms reflected waves, reflection data, reflected energy and reflectors can be used interchangeably and synonymously. In addition, as used herein, the terms refracted waves, refracted energy, refraction energy and refractions are to be understood as equivalent terms.
Three-dimensional seismic surveys are designed primarily to image final drilling objectives ranging from 5,000 feet to more than 18,000 feet below the earth's surface. These three-dimensional surveys are not designed for shallow target resolution.
As current practice in the industry is to conduct shallow hazard marine surveys using reflection data searching for shallow gas-charged zones that would present a danger to the location and structural integrity of off-shore drilling platforms. These surveys are two-dimensional seismic profiles and they are routinely performed today due to the economies of scale involved. A two-dimensional seismic profile is several orders of magnitude less expensive than the capital cost of a deep-water drilling platform and the marine survey can significantly reduce the risk of damage to or loss of the platform.
To date, industry efforts have attempted to employ reflection two-dimensional data and visualization of the reflection data in a three-dimensional display to locate shallow hazards in much the same way as marine two-dimensional surveys have been used. However, it has been found that onshore hazard surveys are more problematic, particularly in shallow carbonate sequences due to near-surface effects and the environmental noise contamination of the seismic data.